Usually terephthalic acid is produced by a liquid phase oxidation of p-xylene and/or p-toluic acid in a solvent comprising an aliphatic carboxylic acid such as acetic acid. Terephthalic acid is of great commercial importance and is widely used for the production of various different polymers, such as fiber-forming polyesters. A process for preparing polyesters of terephthalic acid, particularly polyethylene terephthalate, comprises a direct condensation of terephthalic acid with the respective polyalcohol. For example, terephthalic acid is reacted with ethylene glycol to form bis(.beta.-hydroxyethyl) terephthalate which is then polymerized in a second stage. This direct condensation process is simpler than other known methods such as transesterification of dimethyl terephthalate with the appropriate glycol. However, the direct esterification can desirably require the use of highly purified terephthalic acid. In order to be suitable for the production of polyester fibers, terephthalic acid must be substantially free of any contaminants which lower the melting point of the polyester and/or cause coloration of the polyester. In fact, some impurities which are contained in crude terephthalic acid are color-forming precursors. Additionally, some impurities act as chain terminators in the process to prepare polyesters.
All these impurities have not yet been identified. However 4-carboxybenzaldehyde which is an intermediate oxidation product and which in the following is abbreviated as 4-CBA, generally is found in crude terephthalic acid. It is known that the degree to which coloration in the polyester is induced is less if the 4-CBA content of the terephthalic acid is low. While pure 4-CBA itself can promote coloring during polymerization, this impurity is a convenient tracer for evaluating the degree to which terephthalic acid has been refined. A process which can reduce the 4-CBA content of terephthalic acid reduces also the content of color-forming precursors.
From U.S. Pat. No. 3,584,039 issued to Delbert H. Meyer, incorporated by reference, it is known that fiber-grade terephthalic acid may be prepared by purifying crude terephthalic acid by means of a reduction procedure. The process is essentially comprised of treating an aqueous solution of crude terephthalic acid with hydrogen in the presence of a supported or unsupported Group VIII metal catalyst, whereby the metal and the support are insoluble in the solution under the working conditions. By this process, the amounts of 4-CBA and other coloring impurities contained in terephthalic acid are reduced by formation of removable products. Purified terephthalic acid is then recovered by crystallization, filtration to recover the crystalline product and drying.
As noted above, the oxidation of p-xylene is in the presence of an aliphatic carboxylic acid such as acetic acid as solvent. However, the aliphatic carboxylic acid must be removed before hydrogenation of the crude terephthalic acid. The aliphatic carboxylic acid can act as a poison for the hydrogenation catalyst.
Methods have been proposed for replacement or extraction of the acetic acid from the oxidation effluent with water. For example, U.S. Pat. No. 3,839,436 teaches contacting an oxidation slurry with water, wherein water is introduced into the bottom of a displacement zone, to contact the oxidation effluent in a vertical chamber to effect precipitation of the product acid through the column of water and to remove an aqueous slurry suitable for catalytic purification from the bottom of the column. In another example, European Patent Application, Publication No. EPO 321 272 A1, teaches a process for exchanging or dispersing a medium of a terephthalic acid slurry by introducing an aliphatic carboxylic acid slurry of terephthalic acid into a multi-stage column at the upper part thereof and introducing water at the lower part thereof to form an upflowing stream of water in the multi-stage column, while a sedimentation of terephthalic acid particles is effected inside the multi-stage column, and withdrawing an aqueous aliphatic carboxylic acid solution from the upper part of the multi-stage column, and an aqueous slurry of terephthalic acid from the lower part of the multi-stage column. In the one example, the aqueous slurry of terephthalic acid contained 30,000 ppmw of acetic acid.
These methods suffer from the problem that they do not comprise a positive method of displacing acetic acid from the acetic acid slurry of crude terephthalic acid but rely upon precipitation or sedimentation of the crude terephthalic acid through a column of water.
In another method, International Patent Application No. PCT/JP 89/00529, International Publication WO 89/11323 teaches washing terephthalic acid crystals in a rotary vacuum filter wherein a slurry of terephthalic acid is filtered with suction, the slurry and filtrate being maintained under pressure to keep the temperature and pressure at levels which prevent the filtrate liquor from being supersaturated and thus clogging the filter member. Suction filtration is performed and the terephthalic acid crystals are lifted out of the slurry, as filter cake as the filter member rotates, the slurry being located at the bottom of the filter, the filtrate also being located at the bottom of the filter. The filter cake is thereupon washed in a washing area at the top of the rotating filter drum, the washing solution falling downward into a storage area. The wash solution is sprayed onto the filter cake, falling downward into the storage area, and draining with suction. Positive pressures applied to the slurry and filtrate are at relatively low levels, ranging from 0.5 kg/cm2abs to 5.5. kg/cm2abs. Temperature is in the range of from 90.degree. to 150.degree. C.
This method of washing filter cake suffers from the problems that suction, or vacuum, filtration tends to limit the pressure which can be applied to the process to atmospheric pressure. Spray washing tends to cause channeling of the filter cake with lessened penetration of the filter cake by the washing solution.
It is therefore an object of this invention to provide a method for displacing acetic acid from a slurry of crude terephthalic acid in an acetic acid medium wherein the acetic acid is displaced from the slurry of crude terephthalic acid by a positive displacement method using pressure filtration of the crude terephthalic acid slurry in a method of counter-current flooded water washing, also termed plug flow washing, of the filter cake, followed by reslurrying of the crude terephthalic acid in an aqueous medium suitable for a reduction process in the presence of a Group VIII metal catalyst. The concentration of acetic acid retained in the filter cake is equal to or less than 5000 ppmw.
It is an object of this invention to provide a method for displacing acetic acid from a slurry of crude terephthalic acid in an acetic acid medium wherein the acetic acid is displaced from the slurry of crude terephthalic acid by a positive displacement method using filtration of the crude terephthalic slurry wherein the concentration of acetic acid retained in the filter cake is equal to or less than 5000 ppmw without the need of a drying process to remove the acetic acid in the filter cake by evaporation through application of heat.
It is further an object of this invention to provide an improved process for preparation of crude terephthalic acid by oxidation of an alkyl aromatic in an acetic acid medium wherein the crude terephthalic acid prepared thereby has a concentration of retained acetic acid equal to or less than 5000 ppmw in the absence of application of heat to drive off retained acetic acid from the crude terephthalic acid product.